In a semiconductor process, an apparatus for processing a substrate with a process gas, for example, a film forming apparatus, is often required to uniformly mix plural kinds of process gases and supply the mixture to a substrate. As a method of forming a film on a semiconductor wafer (hereinafter, referred to as a “wafer”) as a substrate, a method of sequentially supplying a raw material gas and a reaction gas reacting with the raw material gas to the wafer to deposit a molecular layer of a reaction product on a surface of the wafer so as to obtain a thin film, a so-called atomic layer deposition (ALD) method or the like, is known. In the ALD method, a first gas and a second gas, which are film forming gases, are alternately supplied in a state in which a carrier gas flows. In order to obtain desirable in-plane uniformity of the wafer in the film forming process, it is necessary that the film forming gases are supplied to the substrate in a state of being uniformly mixed with the carrier gas.
As methods of mixing such a plurality of gases, for example, there is known a technique of uniformly mixing a first gas flow and a second gas flow as a spiral gas flow mixture. Further, there is known a technique of allowing a gas to flow to one side in a circumferential direction, when viewed from a gas outflow passage, and then bending it to be perpendicular to the gas outflow passage side to allow the gas to flow toward a periphery of the gas outflow passage so as to form a swirl flow. In addition, there is known a technique of supplying a raw material gas to a central part of a gas mixing part and uniformly supplying a dilution gas from its periphery in the same flow direction as that of the raw material gas to pass through a gas diffusion part so as to uniformly diffuse the raw material gas and mix them.
However, these known mixing techniques are unsuitable for reducing gas bias in a wide range of flow ratio. In particular, when a small flow rate of the film forming gas is supplied to a large flow rate of the gas, for example, a carrier gas, it is difficult to promote equalization of gases. Further, when a static mixer is installed in the gas mixing device to forcibly mix gases, the pressure loss is increased in the gas mixing device. Since, in order to sequentially supply plural kinds of gases, while substituting the interior of a processing vessel by a purge gas as in the ALD method, it is necessary to allow a large flow rate of the gas to flow at a high speed, using the static mixer is not suitable for the ALD method.
In addition, a small amount of gas is easily mixed by making a flow path of mixed gas long, but when the flow path of mixed gas is long, the temperature of the gas is lowered, and for example, in a certain type of precursor, liquefaction may occur due to a decrease in temperature to cause the generation of particles. Thus, it is required to shorten the flow path of the mixed gas as much as possible.